Recently, it is required to drastically improve an efficiency in the use of energy for automobiles, resulting from energy issues and environmental issues. In order to contribute to the accomplishment through weight reduction, a requirement in which metal parts are replaced by resin parts is growing remarkably.
Recently, as a thermoplastic resin composition used for an automobile member, resins compositions are known such as (heat-resistant)acrylonitrile-butadiene-styrene resins (ABS), alloys of an aromatic polycarbonate resin (PC) and an ABS resin (PC/ABS), and polypropylene resins (PP) filled with talc or glass fiber. These are inexpensive and have a good impact resistance at a low temperature, and have been usefully used. However, many of them have an insufficient rigidity, and thus it is necessary to increase a wall thickness of an article, in order to prevent a deformation caused by a load applied to the members during practical use. In a use to be exposed to severe sunlight irradiation, such as a spoiler, deformation may occur when a temperature of the member is elevated by the sunlight, and thus the resin composition may not be suitable to such a use.
Alloys of PC and an aromatic polyester resin are known as a material having an appropriate rigidity and a sufficient heat resistance in which a material does not deform even if a temperature of the member is elevated under severe sunlight irradiation. For example, an alloy of PC and a polyethylene terephthalate resin (PET) (PC/PET) and an alloy of PC and a polybutylene terephthalate resin (PBT) (PC/PBT) have been proposed. Of these, compositions based on PC/PET are proposed, because it is easy to obtain a large-sized article having inhibited warping during molding from the composition (Patent Documents 1 to 3). When the large-sized molded article formed from the PC/PET composition is assembled to a metal chassis of an automobile, strains are caused by difference in a coefficient of linear expansion due to temperature (a coefficient of linear thermal expansion) between the metal and the PC/PET composition, or the expanded PC/PET molded article is brought into contact with the metal portion at a high temperature to generate scratches. To avoid the defect described above, a design may sometimes be restricted. In order to solve the problems and to realize a molded article having a thin wall thickness to promote the weight reduction, a resin composition containing PC, PET, mineral filler, and bisol-modified PET is proposed (Patent Document 4). A molded article formed from this resin composition has the sufficient rigidity and the excellent resistance to the deformation caused by a load or heat in the practical usage environment, while it has a low coefficient of linear thermal expansion and whose wall thickness is reduced and thus whose weight is also reduced. However, when a part formed from the molded article is coated, the resin composition cannot stand a baking temperature applied to a melamine resin coating material used in base coating and clear coating process of automobile bodies (metal panel, and the like), i.e., 140° C. or higher, because the resin composition has a heat-resistant temperature of about 120 to 130° C. For that reason, it is necessary to use a different coating material capable of being baked at a low temperature of 120° C. or lower and different processes when the coating is performed. In that case, the coating system of the automobile bodies is different from the coating system of the parts formed from the molded article, and thus it is necessary to adjust colors of the coating materials to eliminate the slight color difference occurring between the two, but such an adjustment is quite difficult and the adjustment may sometimes be insufficient when a high design property is required. Even if the initial color adjustment is good, the color change with time on the automobile body is different from that on the part formed from the molded article and the colors vary when they are exposed to an outdoor environment for a long period of time, and thus the adjustment may also sometimes be insufficient when a high design property is required.
Patent Documents 5 and 6 disclose resin compositions in which a polystyrene (PSt) and rubber are admixed with a base of an alloy (PA/PPE) of polyamide resin (PA) and polyphenylene ether resin (PPE) to improve the impact strength, or a needle-like filler is admixed with the PA/PPE to reduce the coefficient of linear thermal expansion. The resin compositions above have a high heat-resistant temperature, and, in a molded article thereof, the thermal deformation is inhibited at a higher temperature of 140° C. or higher. A method to eliminate the occurrence of the color difference between a part and an automobile body is, accordingly, proposed in which a part, obtained by molding a resin composition, is previously assembled to an electrodeposition-coated metal automobile body to integrate them, and then the resulting article is subjected to a generally called “in-line coating” in which an article is coated with a coating material and the baking is performed. In this method, a high hygroscopic property of the resin composition containing PA as the main component may sometimes cause problems as follows. In dry areas such as Europe, the dimension can be stably maintained and the composition distributes the solution of the problems described above, but in humid areas such as Southeast Asia, the deformation caused by moisture absorption easily occurs. To cope with this, the number of joints between the automobile body made of metal and resin parts should be increased, or in order to prevent the generation of scratches, caused by the contact with the metal part, a large gap is required between the metal part and the resin part. The following problems may occur such as occurrence of a failure caused by moisture absorption during the coating, and occurrence of a defect by a poor adhesion with a coating layer because PA forming a continuous phase is a crystalline resin.
Similarly, in order to solve the occurrence of the inferiors caused by the humidity change described above or the coating defect caused by the continuous phase made of the crystalline resin by aiming at elevating the heat-resistant temperature, a resin composition is proposed in which a highly heat-resistant polycarbonate resin having a specific alicyclic structure in its molecule, PET, an elastomeric polymer, filler and/or a reinforcing material are used (Patent Document 7). It is assumed that a part obtained by molding the resin composition is subjected to an in-line coating as the case of the PA/PPE. Although Patent Document 7 does not clearly describe what kind of a base resin is used in the coating material when the in-line coating is performed, according to Patent Document 7, the part obtained from the resin composition can keep a surface smoothness thereof good, and a coating surface with an excellent appearance can be obtained, even after the in-line coating. When a baking process using a melamine resin coating material is accompanied, however, the heat resistance of the resin composition is insufficient, and the part itself may sometimes be deformed at a high temperature, and a shape of the design or a size of the part may sometimes be restricted. In a case of large-sized outer panel, the difference in the coefficient of linear thermal expansion with the metal site becomes a problem, and there is a restriction in terms of the design; for example, it is necessary to make a large gap for avoiding interference, and improvements are required.